Invasive species are animals, plants, parasites or disease-causing organisms that establish themselves outside their natural range and become pests. Although native species can also become invasive if transferred outside their natural range (DEH 2004), this report does not include native species in its discussion of invasive species.

Invasive species have a major impact on Australia’s environment, threatening biodiversity, and reducing overall species abundance and diversity. They represent one of the more potent, persistent and widespread threats to the environment, and are the most frequently cited threat to EPBC Act–listed species.

Native species are directly affected by invasive species through predation, displacement, competition and hybridisation. Invasive species can also have enormous harmful effects on the health, viability and functioning of ecological communities, ecosystems and landscapes, through both direct and indirect disruption of ecological services such as soil stabilisation, pollination and seed dispersal, and effects on fire frequency and intensity. They alter habitat and reduce biodiversity in both land and marine environments, and can adversely affect the recreational, social and commercial value of ecosystems.

Although there are insufficient data to robustly assess the abundance and trends of most invasive animals and invasive plants, many that are not subject to active control and/or eradication programs appear to be increasing in their distribution and abundance.

Invasive animals

Invasive species, including cats (Felis catus), rabbits (Oryctolagus cuniculus), goats (Capra hircus), rats (Rattus spp.), cane toads (Rhinella marina), foxes (Vulpes vulpes), feral pigs (Sus scofa), escaped or dumped domestic animals or pets, red imported fire ants (Solenopsis invicta) and yellow crazy ants (Anoplolepis gracilipes), are listed as the cause of decline in native species and/or ecological communities in 12 of the 21 key threatening processes listed under the EPBC Act.

Predation by cats and red foxes has contributed the most to extinction of mammals in Australia, and this pressure continues to contribute to the decline of threatened mammals (Woinarski et al. 2015). For example, a recent analysis of the diet of feral cats recorded 400 vertebrate species (native and non-native) that feral cats feed on or kill in Australia (Doherty et al. 2015). These included 17 EPBC Act–listed species, 123 birds, 157 reptiles, 58 marsupials, 27 rodents, 5 bats, 21 frogs, and 9 medium-sized and large exotic mammals. Cats also consume a wide range of insects, spiders, scorpions, centipedes and crustaceans.

Other significant animal invaders include cane toads and rabbits. Cane toads have been reported in the Kimberley and upper reaches of the Fitzroy River in Western Australia (Pusey & Kath 2014), and inland western Queensland, with estimates of expansion of their range of around 10–50 kilometres per year. Poisoning by cane toads is a major threat for 4 species of threatened mammals—for example, cane toads significantly affect populations of the northern quoll (Dasyurus hallucatus) in northern Australia.

In the past, the introduced black rat (Rattus rattus) has contributed to the extinction of several mammal species, as well as some island subpopulations, through predation, competition or disease transmission. Today, it is considered a major threat for a number of other threatened species in The action plan for Australian mammals 2012 (Woinarski et al. 2014a). Black rats have invaded bushland areas around most of Australia’s major coastal cities, often replacing native mammals that have become locally extinct.

Invasive animals that put pressure on inland waters include cane toads, common carp (Cyprinus carpio), eastern gambusia (Gambusia holbrooki) and goldfish (Carassius auratus). Approximately 43 invasive freshwater fish species have established wild populations, 34 of which continue to spread. This number is uncertain, because survey data are scarce and unevenly distributed across aquatic ecosystems.

The distribution of carp throughout inland waters in Australia is similar to that reported in 2011. Although the eastern gambusia is one of the more widespread invasive freshwater fish species in Australia, with sightings in all mainland states and territories, data from the Atlas of living Australia show no significant expansion in the species distribution since 2011.

Several introduced species are widespread in the marine environment, including the New Zealand screw shell (Maoricolpus roseus) (Gunasekera et al. 2005) and the northern Pacific starfish (Asterias amurensis) (Ross et al. 2003). These species were likely introduced through biofouling of ship hulls or ballast discharge.

Invasive plants

The list of Weeds of National Significance was updated in 2012, and 12 new weeds were added (bringing the total to 32), including 2 aquatic species: sagittaria/arrowhead (Sagittaria platyphylla) and water hyacinth (Eichhornia crassipes) (Australian Weeds Committee 2012). An additional 28 non-native weeds that have established naturalised populations in the wild have been added to the National Environmental Alert List. These are species that are in the early stages of establishment and have the potential to become a significant threat to biodiversity if they are not managed (DoEE 2016a).

Invasive plants continue to have a negative impact:

• on the productivity of Australian agriculture and forestry
• on the natural environment through impacts on biodiversity, ecosystem function and environmental health, and through promotion of bushfires
• on access to sites of significant Indigenous cultural heritage
• in some cases, on public health through toxicity, allergic reactions and respiratory diseases.

An emerging issue in all cropping areas of Australia is herbicide-resistant weed populations. In June 2014, at least 39 weed species in Australia were resistant to one or more herbicides, and the number of identified resistant species is growing. Herbicide-resistant weeds pose a potential threat to native vegetation communities and agricultural crops, and also threaten the viability of some no-till farming systems that are designed to limit soil disturbance, and thus loss of soil and nutrients through erosion (GRDC 2016).

Invasive pathogens

Invasive pathogens can cause widespread mortality, habitat loss and degradation of ecosystems. Although Australia is free from many of the most damaging agricultural plant pathogens because of concerted biosecurity efforts at all levels of government, a few significant pathogens have become established or are near our borders. Most of these are of potential threat to commercially grown species, but many can also affect native species.

Three key threatening processes listed under the EPBC Act relate to pathogens: the chytrid amphibian fungus (Batrachochytrium dendrobatidis); the root rot pathogen Phytophthora cinnamomi; and psittacine circovirus, which causes beak and feather disease (psittacine circoviral disease) in parrots.

Three pathogens identified in SoE 2011 remain a particular concern in 2016: the chytrid fungus, the root-rot pathogen and myrtle rust (Puccinia psidii).

The chytrid fungus caused rapid decline in native amphibian populations in Australia and the Americas in the 1970s. Six Australian amphibian species have become extinct since the first documented occurrences of chytridiomycosis, and another 7 are at high risk of extinction (Skerratt et al. 2016).

The area of native vegetation affected by root-rot now exceeds 1 million hectares in Western Australia, many hundreds of thousands of hectares in Victoria and Tasmania, and tens of thousands of hectares in South Australia. South Australia reports that root rot is becoming more widespread in that state.

The invasive myrtle rust, first detected in Australia in New South Wales in 2010, is now established in the natural ecosystem throughout coastal New South Wales, and south-east and far north Queensland; it has a more limited distribution in Victoria, Tasmania and the Northern Territory (Carnegie et al. 2015). Myrtle rust is of particular concern because its host plant family, the Myrtaceae (which includes Eucalyptus and Melaleuca), is a core component of Australia’s vegetation and a key driver of ecological processes. Many industries, such as forestry, nursery, essential oils and cut flowers, are reliant on myrtaceous species.

Based on reporting of events since 2011, there has been no change in the frequency of occurrence of major viral diseases, parasitic infestations or mass die-offs in marine and coastal waters, and conditions remain stable at a national level. An outbreak of Pacific oyster mortality syndrome in 2016 in Tasmania was the first reported for the state and confirmed the establishment of the causative virus in Tasmanian waters.